Measuring apparatuses are known having means for digitally removing errors from a measurement system by converting one or more measurements to a digital value or values and then applying correction algorithms to the digital value or values. These algorithms make use of a table or tables of previously stored digital data which describe the corrections required to make the measurement system more accurate. These tables are stored in the measurement system, and effectively describe the errors of the complete measurement system including a transducer, amplifiers, filters, and analog to digital (A/D) converters.
FIG. 1 shows a typical prior art circuit in which temperature measurements are converted to a voltage 17 by a transducer 16. The voltage 17 is applied to calibrated measuring system 100. After amplification and filtering 10, the internal signal is converted to a digital value by an A/D converter 11. Errors may be introduced by the transducer characteristics, amplifier and filter offset and gain errors, and A/D converter linearity and gain errors.
A microprocessor 13 then looks up this uncorrected digital value 12 in a table of uncorrected values vs. corrected digital value 14. The corrected value can be provided to a user in the display 15. The microprocessor may apply more complicated correction algorithms, such as interpolation and data transformations, to reduce the amount of data stored in the table or to increase the accuracy of the result.
In addition to increasing the accuracy of the measurement system, this prior art digital calibration makes it easier to automate the manufacturing process for the measurement system. Calibration can be performed by a completely automated system which applies a series of known values to the transducer, examines the uncorrected output of the measurement system under calibration, and builds a table of corrections which can then be stored in non-volatile digital memory in the measurement system.
FIG. 2 shows a graphical representation of the table of corrections 14 comparing the uncorrected amplitude 200 to corrected amplitude 201. If the system introduces no errors the uncorrected data 12 will fall on the dotted line 202. Offset errors are shown as the line 203, gain errors as the line 204 and non-linearity errors as the line 205. A more complete discussion of such systems may be found in the background section of our co-pending application entitled METHOD AND APPARATUS FOR CALIBRATING TRANSDUCER/AMPLIFIER SYSTEMS, Ser. No. 308,305 now U.S. Pat. No. 5,014,229.
Because the table of corrections 14 describes the complete system, replacing any element of the system including the transducer requires re-calibrating the measurement system. Certain prior art systems include the transducer 16 in the calibration procedure while others calibrate the remainder of the system 100 ignoring errors introduced by the transducer 16. This may be impractical, especially if many plug-in transducers are desirable. Such a system may be desirable, for example, in a temperature sensing system having several probes, each of which is sensitive over different ranges of temperatures.
Some prior art systems ignore the errors of plug-in transducers and only calibrate the remaining parts of the measurement system; this approach can leave in place substantial transducer errors. Other prior art systems use analog calibration of plug-in transducers; this approach makes it more difficult to automate the production of the entire measurement system.